1. Field of the Invention
The present invention relates generally to a circuit for detecting digital color picture movement signals in television receiving equipment (TVs), video tape recorders (VTRs), and so on and relates in particular to a picture movement signal detecting means and its associated circuit.
2. Description of the Related Art
The conventional method of improving picture quality in digital color television receiving equipment and video tape recorders comprises the steps of detecting the motion of the picture by investigating correlations which might exist between different frames in the television picture signal, separating the brightness and color signals, usually referred to as "YC separation," between different frames for a picture with relatively little motion and executing the YC separation within the picture field under consideration in the case of a picture with an abundance of motion. YC separation can be carried out by either line-to-line separation or by frequency separation. A number of picture signal detection circuit arrangements for detecting the picture movement signal, e.g. circuit arrangements using either a set of frame memories which is capable of memorizing information of an amount equivalent to one frame (two fields), 1.5 frames (three fields) and two frames (four fields) are proposed and applied.
In general, it is clear that the more frame memories used, the more precise the detection that can be expected. However, since a frame memory is usually expensive, a detecting circuit using a frame memory of only one frame length and capable of detecting the picture movement signals precisely is desired.
FIG. 1 is a block diagram of an example of a conventional picture movement signal detecting circuit which emphasizes detecting picture movement signals and which uses a frame memory capable of storing one frame length of the color picture signal and a pair of comb filters.
An input signal 301 is a digital color picture signal digitalized by an analog to digital (A/D) converter from a conventional NTSC analog television video signal and is produced by mixing the brightness (Y) and color (C) signals by interleaving, as is usual. The input digital color picture signal 301 is converted to a brightness signal by removing its color signal component by means of a comb filter which consists of a pair of line memories 302, 303, and a pair of adders 304, 305 and which will hereafter be referred to as the first comb filter. The adders in turn serve to add 1/2 of the signal representing the line under consideration and 1/4 each of the signals representing the preceding and the succeeding lines, outputting the brightness signal component contained in the input digital color picture signal 301 without the color component. This is possible because the phase of each adjacent line of NTSC color signals differs by 180.degree.. On the other hand, the input digital color picture signal 301 is converted to a brightness signal delayed by one frame period by inputting through the frame memory 306 and the second comb filter which consists of a pair of line memories 307, 308 and a pair of adders 309, 310.
The differential signal is then output by subtracting the output signals from the two comb filters through the subtracter 311. The differential signal contains only the movement component of the brightness signal. However, when there is no correlation between lines, leakage of a very small amount of the color component from the comb filters is unavoidable. Accordingly, a low-pass filter 312 is provided for removing residual color component to output a pure picture movement signal 313. Unfortunately, however, the low-pass filter 312 also removes the high-frequency component of the brightness movement signal, with the result that a conventional system as explained above cannot distinguish the picture movement signal from the original input digital picture signal in cases when the picture consists of narrow vertical stripes moving horizontally at a rapid rate.